Copper and zinc oxide-based catalysts have been known for many years; they have been described as soon as 1933 by DODGE (U.S. Pat. No. 1,908,696). In U.S. Pat. Nos. 3,388,972 , 3,546,140 and 3,790,505 , the U.S. company C.C.I. describes the use of Cu, Zn, Al ternary compositions for the conversion at low temperature of carbon oxide (CO) and for the synthesis of methanol.
Various methods for preparing Cu, Zn, Al catalysts are especially described in U.S. Pat. Nos. 3,923,694 , 4,279,781, 4,596,782 and FR-A-2,352,588.
FR-A-2,352,588 describes the preparation of a catalyst through the mechanical mixing of 10 to 60% of copper oxide, 5 to 40% of zinc oxide and 30 to 70% of an aluminous cement, as well as the use of this catalyst for the conversion of the carbon oxide in order to produce hydrogen or methanol.
Apart from the catalyst described in FR-A-2,352,588, these catalysts are generally produced by a precipitation reaction between an acid solution containing for example the nitrates of the metals cited above and a basic solution containing for example an alkaline carbonate. The precipitation reaction, such as that which is for example described in U.S. Pat. No. 3,923,694, leads to obtaining hydrated precursors, at least partly crystallized, consisting of at least three phases: a CuAlZn ternary phase, with (Cu+Zn)/Al=3 (atoms.atom.sup.-1), of the hydrotalcite type; a binary phase, the rosacite, which is a mixed copper and zinc hydroxycarbonate; and a copper hydroxycarbonate, the malachite, and possibly also other phases, such as for example the ZnAl.sub.2 O.sub.4 spinel described in U.S. Pat. No. 3,923,694. The consequence of the heterogeneousness of composition of these catalysts is relatively low activity, selectivity and stability, even though they initially contain a copper oxide that is well-dispersed, at least partly. Various publications, as for example F. TRIFIRO et al, Preparation of Catalysts III, p. 723-733, 1983, Elsevier Science Publishers (Amsterdam), give a detailed description of the simultaneous forming of these phases.
The preparation of copper-based catalysts obtained by the calcination of identified crystallized precursors, and more particularly of hydroxycarbonated phases obtained by co-precipitation, has been rarely described. U.S. Pat. No. 4,145,400 describes the preparation of CuZnAl catalysts, but said catalysts are prepared from only one crystallized monophase precursor, hydrotalcite. As for U.S. Pat. No. 4,596,782, it describes the preparation of a methanol synthesis catalyst from only one amorphous hydrated precursor with a very high homogeneity. On the other hand, the catalyst described in U.S. Pat. No. 4,436,833 is prepared from the mixture of a binary crystallized phase of formulation Cu.sub.2,2 Zn.sub.2,8 (OH).sub.16 (CO.sub.3).sub.2 and of aluminum hydroxide. Lastly, U.S. Pat. No. 3,923,694 describes a sequential co-precipitation where a spinel precursor containing aluminum and zinc is first obtained, then a binary Cu-Zn compound is co-precipitated on this precursor. In numerous documents, for example U.S. Pat. Nos. 3,388,972, 3,546,140, FR-A-2,027,162, the alumina, which is the main component of the catalyst, is introduced in the oxide state or else in the hydroxide state (U.S. Pat. No. 4,279,781).
The precursor, preferably hydroxycarbonated, can be obtained according to the invention by mixing together two ternary precursors, preferably hydroxycarbonated, each comprising at least copper, aluminum and zinc, one of said ternary precursors containing at least 50%, preferably at least 65%, and, more preferably, at least 85% by weight of a phase called "roderite" defined hereafter, and the other one of said ternary precursors containing at least 50 preferably at least 65%, and more preferably, at least 85% by weight of a phase called "prespinel" defined hereafter.
It seems that the roderite phase has already been examined, but badly characterized by DOESBURG E.B.M. (Univ. DELFT), HOPPENER (DSM, Studies in Surface Science and Catalysis Vol. 31, p. 767, 1987); still the same authors declare that the most active catalysts do not contain this phase, but a mixture of the rosacite and hydrotalcite phases. Moreover, one of these phases (rosacite) merely consists of the two elements copper and zinc, contrarily to the two ternary precursors used in the present invention, which contain the three elements Cu,Zn,Al. The prespinel phase has not been described to date.
As shown through the microanalyses by X-ray emission spectrometry carried out by the applicant, the elemental roderite and prespinel phases show a different ternary composition in Cu, Zn and Al:
the roderite phase shows an atomic ratio (Cu+Zn)/Al ranging from 3.5 to 16 and preferably from 4 to 13, and an atomic ratio Zn/Al ranging from 1.6 to 6 and preferably from 2 to 5 ; PA1 the prespinel phase shows an atomic ratio (Cu+Zn)/Al ranging from 0.20 to 2.10 and preferably from 0.25 to 1.50, and an atomic ratio Zn/Al ranging from 0.10 to 1.50 and preferably from 0.15 to 1.20.
Said elemental phases, the precursors and the catalysts of the present invention can be characterized by an X-ray emission spectrometry in a scanning transmission electron microscope. The analyses are carried out for example by means of a device which can give high-resolution images (0.5 to 1 nm) in the STEM mode and shows a high sensitivity in the X-ray micro-analysis mode. A commercial device such as the scanning transmission electron microscope Vacuum Generator HB 501, equipped with a Si-Li KEVEX detector associated with a TRACOR analyzer, is quite suitable (sensitivity limit higher than 1,000 atoms of a given element) for determining the morphology and the local composition of the precursors and catalysts. In order to preserve the organization of the phases between each other and to describe their macroscopic distribution in the co-precipitate grains during the preparation of the precursors (see hereafter) better, a preparation method using an ultramicrotom cutting in order to obtain grain sections with a thickness of several tens of manometers (1 nm=10.sup.-9 m) can be used.
Thus, after selecting the zone to be analyzed (typically 2 to 5 nm), several countings of a duration of 100-1000 s, leading to a sufficiently precise counting statistic (over 10%), are simultaneously performed.
From the intensities that are measured on the various peaks selected for the different elements that are present in the sample, it is possible to determine their relative concentrations, then their respective atomic ratios, according to well-known X-ray emission techniques (see for example REED S.J.B. Electron microprobe Analysis, Cambridge University Press, 1975), for each of the particles constituting the sample.
The samples that are compared must all show the same thickness. The average values of the coefficients of correction (reduced to Cu-K .alpha.=1) are the following:
______________________________________ Line measurements Element Coefficient ______________________________________ K.alpha. copper 1 K.alpha. aluminum 4.86 K.beta. zinc 4.68 ______________________________________
These coefficients have been determined by the applicant from mixed oxides calcined at a high temperature: EQU CuAl.sub.2 O.sub.4, ZnAl.sub.2 O.sub.4, Cu.sub.1-x Zn.sub.x Al.sub.2 O.sub.4 (x=0.25-0.50-0.75)
are the reference samples.
The atomic ratio Zn/Al is for example calculated as follows (iK.beta. Zn and iK.alpha. Al are the average rough intensities determined through several countings): EQU Zn/Al=0.63 i.sub.K.beta. Zn/i.sub.K.alpha. Al.
The best results, in terms of activity, selectivity and stability, are generally obtained with catalysts for which each monophase (roderite, prespinel) shows a variation of the atomic ratios (Cu+Zn)/Al and Zn/Al lower than about 15% and preferably lower than about 10% in relation to the average value of this ratio, at the scale of 5 manometers.